With recent performance improvement and size reduction of an image pickup device using a solid-state imaging element such as a CCD (Charge Coupled Device) type image sensor or a CMOS (Complementary Metal Oxide Semiconductor) type image sensor and the like, a mobile phone and a portable information terminal provided with an image pickup device has begun to spread in recent years. Since restriction on size and cost is extremely strict in these devices, an image pickup device provided with a small-sized solid-state imaging element with the number of pixels smaller than that of a normal digital still camera and the like, and a single focus optical system composed of 1 to 4 plastic lenses is generally used. In rapid progress of higher pixelation and higher functions of the image pickup device mounted on the portable information terminal and the like, a small-sized zoom lens not only capable of handling a high-pixel image pickup element and of picking up an image of an object apart from a photographer but also capable of photographing even if a distance from the object cannot be set away as in indoor photographing and of being mountable on a mobile phone and the like is in demand.
In order to mount a zoom lens on a mobile phone or a portable information terminal, reduction in size or particularly in a thickness direction is in demand. As a zoom lens of this thin type, a bending optical system in which an optical axis is bent by 90 degrees by using a catoptric element such as a prism and the like is used in many cases. Specifically, a variable-magnification optical system with the purpose of size reduction in a thickness direction by using the catoptric element in a first lens group is disclosed in the following Patent Literatures 1 to 4.
In general, a variable-magnification optical system having the first lens group having a negative refractive power and a second lens group having a positive refractive power as in Patent Literatures 1 to 4 is advantageous in points that the thickness of the first lens group can be made small, that the number of constituting lenses can be made small, and that F-number of a wide-angle end can be made bright or fast, but since the first lens group has a negative refractive power, an effective diameter of the second lens group tends to become larger, and there is a problem that an increase in the effective diameter in a bending optical system portion directly leads to an increase of the thickness. Particularly, the second lens group is often accompanied with an aperture stop and is a spot where an axial ray passes thickly, and thus, a thinning method used in the bending optical system portion in many cases in which a lens shape obtained by cutting a area not in use or a so-called oval shape is used instead of a circular shape symmetrical to the optical axis cannot be employed.
Moreover, since the second lens group has a large light beam diameter passing through the lens and moves for a long distance during magnification change from the wide-angle end to a telephoto end, decentration error sensitivity of the second lens group needs to be made as small as possible. Thus, when the second lens group is composed of a plurality of lens elements as in Patent Literatures 1 and 2, axial coma aberration or asymmetrical blur in an image area called one-side blur generated in the second lens group is reduced in many cases by centering a partial lens or lenses in the second lens group. However, since the centering is an action for reducing axial coma aberration or one-side blur by decentering the lens with respect to the optical axis, a space needs to be ensured for centering in the second lens group, and there is a problem of size increase of the second lens group and hence thickening of a variable-magnification optical system.
On the other hand, in Patent Literatures 3 and 4, by constituting the second lens group by one lens element, the total thickness of the lens is reduced, and optical adjustment such as centering is no longer necessary, and thus, the effective diameter of the second lens group can be made small. However, in the variable-magnification optical system in Patent Literature 3, since the second lens group is constituted by one cemented lens of a lens having a positive refractive power and a lens having a negative refractive power, an aberration correcting capacity is low, and as a result, F-number becomes dark or slow. Moreover, in Patent Literature 4, the second lens group is constituted by a cemented lens of a lens having a positive refractive power, a lens having a negative refractive power, and a lens having a positive refractive power in order from the object side, but since freedom of a lens surface is made small due to the cementing, a burden on an optical surface in contact with air becomes large, and the decentration error sensitivity of the surface becomes extremely high.